Magnetic tunneling junction film structure with process determined in-plane magnetic anisotropy

ABSTRACT

A method of forming an MTJ memory cell and/or an array of such cells is provided wherein each such cell has a small circular horizontal cross-section of 1.0 microns or less in diameter and wherein the ferromagnetic free layer of each such cell has a magnetic anisotropy produced by a magnetic coupling with a thin antiferromagnetic layer that is formed on the free layer. The MTJ memory cell so provided is far less sensitive to shape irregularities and edge defects than cells of the prior art.

This application is related to application Ser. No. 10/872,915, filed onJun. 21, 2004, now issued as U.S. Pat. No. 6,979,586, and is alsorelated to to, a Divisional of the former application, application Ser.No. 11/210,637, filed on Aug. 25, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the design and fabrication of magnetic tunneljunctions (MTJ) as memory storage devices, particularly to a designwherein coercivity and anisotropy are decoupled from the cell shape ofthe junction and can be independently optimized.

2. Description of the Related Art

The magnetic tunnel junction (MTJ) basically comprises two electrodes,which are layers of ferromagnetic material, separated by a tunnelbarrier layer, which is a thin layer of insulating material. The tunnelbarrier layer must be sufficiently thin so that there is a probabilityfor charge carriers (typically electrons) to cross the layer by means ofquantum mechanical tunneling. The tunneling probability is spindependent, however, depending on the availability of tunneling stateswith different electron spin orientations. Thus, the overall tunnelingcurrent will depend on the number of spin-up vs. spin-down electrons,which in turn depends on the orientation of the electron spin relativeto the magnetization direction of the ferromagnetic layers. Thus, ifthese magnetization directions are varied for a given applied voltage,the tunneling current will also vary as a function of the relativedirections. As a result of the behavior of an MTJ, sensing the change oftunneling current for a fixed potential can enable a determination ofthe relative magnetization directions of the two ferromagnetic layersthat comprise it. Equivalently, the resistance of the MTJ can bemeasured, since different relative magnetization directions will producedifferent resistances.

The use of an MTJ as an information storage device requires that themagnetization of at least one of its ferromagnetic layers can be variedrelative to the other and also that changes in the relative directionscan be sensed by means of variations in the tunneling current or,equivalently, the junction resistance. In its simplest form as a twostate memory storage device, the MTJ need only be capable of having itsmagnetizations put into parallel or antiparallel configurations(writing) and that these two configurations can be sensed by tunnelingcurrent variations or resistance variations (reading). In practice, thefree ferromagnetic layer can be modeled as having a magnetization whichis free to rotate but which energetically prefers to align in eitherdirection along its easy axis (the direction of magnetic crystallineanisotropy). The magnetization of the fixed layer may be thought of asbeing permanently aligned in its easy axis direction. When the freelayer is anti-aligned with the fixed layer, the junction will have itsmaximum resistance, when the free layer is aligned with the fixed layer,the minimum resistance is present. In typical MRAM circuitry, the MTJdevices are located at the intersection of current carrying lines calledword lines and bit lines (or word lines and sense lines). When bothlines are activated, the device is written upon, ie, the magnetizationdirection of its free layer is changed. When only one line is activated,the resistance of the device can be sensed, so the device is effectivelyread. Such an MTJ device is provided by Gallagher et al. (U.S. Pat. No.5,650,958), who teach the formation of an MTJ device with a pinnedferromagnetic layer whose magnetization is in the plane of the layer butnot free to rotate, together with a free magnetic layer whosemagnetization is free to rotate relative to that of the pinned layer,wherein the two layers are separated by an insulating tunnel barrierlayer.

In order for the MTJ MRAM device to be competitive with other forms ofDRAM, it is necessary that the MTJ be made very small, typically ofsub-micron dimension. Such a small area cell is provided by Gallagher etal. (U.S. Pat. No. 6,226,160 B1) who make use of a tunnel barrier layerformed of an oxidized thin aluminum layer. Parkin et al. (U.S. Pat. No.6,166,948) notes that sub-micron dimensions are needed to be competitivewith DRAM memories in the range of 10–100 Mbit capacities. Parkin alsonotes that such small sizes are associated with significant problems,particularly super-paramagnetism, which is the spontaneous thermalfluctuation of magnetization in samples of ferromagnetic material toosmall to have sufficient magnetic anisotropy (a measure of the abilityof a sample to maintain a given magnetization direction). It is alsoundesirable for MTJ devices to have excessive magnetic coupling betweenadjacent magnetic layers of neighboring devices or even within the samedevice as this coupling must be overcome when writing on the device. Onesource of such undesirable coupling results from the non-planar surfacesat the interfaces of ferromagnetic layers, such as might occur betweenthe fixed and free layer of an MTJ. This is known as topologicalcoupling. Slaughter et al. (U.S. Pat. No. 6,205,052 B1) teaches a way ofreducing such topological coupling by forming an additional layerbetween a base metal and a spacer layer, the additional layer beingcrystallographically amorphous with respect to x-ray scatteringanalysis.

Some degree of anisotropy is necessary if an MTJ cell is to be capableof maintaining a magnetization direction and, thereby, to effectivelystore data even when write currents are zero. As cell sizes havecontinued to decrease, the technology has sought to provide a degree ofmagnetic anisotropy by forming cells in a wide variety of shapes (eg.rectangles, diamonds, ellipses, etc.), so that the lack of inherentcrystalline anisotropy is countered by a shape anisotropy. Yet this formof anisotropy brings with it its own problems. A particularlytroublesome shape-related problem in MTJ devices results fromnon-uniform and uncontrollable edge-fields produced by shape-anisotropy(a property of non-circular samples). As the cell size decreases, theseedge fields become relatively more important than the magnetization ofthe body of the cell and have an adverse effect on the storage andreading of data. Although such shape anisotropies, when of sufficientmagnitude, reduce the disadvantageous effects of super-paramagnetism,they have the negative effect of requiring high currents to change themagnetization direction of the MTJ for the purpose of storing data. Tocounteract these edge effects, Shi et al. (U.S. Pat. No. 5,757,695)teaches the formation of an ellipsoidal MTJ cell wherein themagnetization vectors are aligned along the length (major axis) of thecell and which do not present variously oriented edge domains, highfields and poles at the ends of the element. In addition, thefabrication processes required to produce the shape varieties, eg.photolithography and ion-milling, are incapable of controlling theshaping with sufficient precision to prevent variations in cell sizes,shapes and aspect ratios and, in addition, cause uncontrollable defectsalong the edges of the cells. The randomness of all these defects leadto a wide distribution in switching field coercivities (the fieldsrequired to change the logic state of an MTJ cell) and can even causeunwanted and uncontrollable coupling between cells. One attempt toreduce edge effects is provided by Nakao et al. (U.S. Pat. No. 6,351,410B1), who form ring shaped MTJ electrodes to cause the induced magneticfields to be circumferential.

As has been discussed, many of the problems associated with theconstruction of MRAM arrays are related to the shapes of the cells andthe processes required to form those shapes. Cell shapes of presentdesigns are typically single element rectangle, elliptical or lozenge.Any irregularities of these shapes, or defects at their edges producedduring their formation, will result in coercivity fluctuationsdistributed throughout the array. An alternative approach to providingmagnetic anisotropies without the necessity of utilizing shapes whichare difficult to fabricate, is to produce a magnetic anisotropy in aferromagnetic layer by forming it on an antiferromagnetic layer. In thisway, a magnetic coupling can be produced between the ferromagnetic andantiferromagnetic layers which will provide the required magneticanisotropy. In “Orientational dependence of the exchange biasing inmolecular-beam-epitaxy-grown Ni₈₀Fe₂₀/Fe₅₀Mn₅₀ bilayers” (R. Jungblut,R. Coehoorn, M. T. Johnson, J. aan de Stegge and A. Reinders, J. Appl.Phys. 75(10), 15 May 1994, pp. 6659–6664), experimental results areprovided to show that interfacial exchange energy between such layerscan be utilized to provide a biasing effect which lowers coercivity (asindicated by hysteresis loop shifts) in crystal growth directions.Fujikata et al. (U.S. Pat. No. 5,766,743) provide a magnetoresistanceeffect film having two ferromagnetic layers separated by a non-magneticlayer wherein one of the ferromagnetic layers is formed on anantiferromagnetic layer. The antiferromagnetic layer is at least partlymade of a NiMn alloy having a face-centered tetragonal structure and itprovides a biasing magnetic field, Hr, which exceeds the coercive force,Hc₂, of the other ferromagnetic layer. The purpose of theantiferromagnetic layer is to provide domain stabilization of the freelayer and, as a result the antiferromagnetic layer is not formed overthe active region of the free layer. In a related invention, Rizzo etal. (U.S. Pat. No. 6,430,084 B1) teach the formation of bit and digitlines (the lines whose currents write and read the MTJ devices) whichare clad with shielding ferromagnetic and antiferromagnetic layers toprevent inadvertent switching of adjacent MTJ cells. In the structureprovided by Rizzo, the antiferromagnetic layer stabilizes themagnetization of the ferromagnetic layer by exchange coupling andthereby improves the shielding effect. It is the object of the presentinvention to utilize the magnetic coupling properties ofantiferromagnetic layers with ferromagnetic layers to provide thenecessary magnetic anisotropy to form an MTJ memory cell capable ofadvantageously storing data without the necessity of obtaining theanisotropy through the route of shape anisotropy.

SUMMARY OF THE INVENTION

A first object of this invention is to provide a novel MTJ device whosemagnetization switching properties are insensitive to shapeirregularities and edge defects and which can be used to form an MRAMarray.

A second object of this invention is to provide an MRAM array of suchMTJ devices, in which array coercivity variations and resultingswitching field variations due to shape irregularities and edge defectsin the MTJ devices is eliminated or greatly reduced.

A third object of this invention is to provide an MRAM cell array designwhich is less dependent on the shape of individual cell elements for itsperformance.

A fourth object of this invention is to provide an MTJ cell which can beformed with a substantially circular shape of sub-micron dimension usingeven square shaped masks for cell patterning.

These objects will be achieved by a design method that decouples cellmagnetization anisotropy from cell shape and cell layer thickness. Inaccord with the method, the cell shape can be circular in its horizontalcross-section, which is a particularly simple shape to fabricate andwhose shape variations can be easily controlled. In fact, for smallenough dimensions (less than 1.0 microns), a simple square mask designwill almost necessarily create a substantially circular cell pattern.The success of the method is based on the fact that a very thinantiferromagnetic material layer (less than 20 angstroms thick) grown ona ferromagnetic material layer can render the magnetization of theferromagnetic layer effectively magnetically anisotropic (before theunidirectional anisotropy is fully developed) within the plane of itsformation by means of magnetic coupling across the interface between thetwo layers. The method proposes that a ferromagnetic free layer of anMTJ device be formed, with a circular shape, beneath a “top” thinantiferromagnetic layer. Although the circular shape will provide noshape anisotropy and although the small size of the free layer willprovide no crystalline anisotropy, the top antiferromagnetic layerformed on the free layer will provide an anisotropic magnetic coupling.Since no shape anisotropy is required, the MTJ cell can be formed withdimensions less than 1.0 microns using even a square mask design, whichat such small dimensions will produce a patterned cell that issubstantially circular. The free layer can be a single layer offerromagnetic material or it can be formed as a synthetic ferromagneticlayer with a top layer of antiferromagnetic material. The magnetizationof the top antiferromagnetic layer will be set in the same direction asthe bottom antiferromagnetic layer that pins the fixed ferromagneticlayer of the MTJ device. The magnetizations will be set in the samemagnetic field and at the same temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 a is a schematic illustration of a single MTJ device formed inaccord with the method of the present invention. The device is shown (invertical cross-section) at the intersection of a word line and a bitline and it is understood that it can be one of a plurality of suchdevices forming a MRAM array.

FIG. 1 b shows an overhead view of FIG. 1 a, indicating thesubstantially circular horizontal cross-section of the MTJ device.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The preferred embodiment of the present invention teaches a method offorming an MTJ device (also called an MTJ memory cell) as one of an MRAMcell array of such devices, the ferromagnetic free layer of each suchdevice having a magnetic anisotropy created by magnetic coupling throughtheir interface with an antiferromagnetic layer.

Referring to FIG. 1 a, there is shown a schematic verticalcross-sectional view of an MTJ cell (10) formed in accord with themethod of the present invention. The cell is disposed between asubstantially planar substrate, which contains a word line (30) that isinsulated (90) from the cell and a bit line (20), which contacts anupper portion of the cell and which runs orthogonally to the word line.A lower conducting electrode (35) contacts a lower portion of the celland is required for reading operations. The cell is thereby positionedat an orthogonal junction between the word and bit lines and is disposedbetween them. An overhead view of the MTJ cell shown schematically inFIG. 1 b discloses the substantially circular horizontal cross-sectionalshape of the preferred embodiment, which is not seen in this verticalcross-section.

Referring to FIG. 1 a, there is shown a ferromagnetic free layer (40)separated by an insulating tunneling layer (50) from a magneticallypinned layer (60). The magnetically free layer is preferably a layer ofCo, Ni, Fe or their alloys, CoFeB, CoZrB, CoTaB or CoHfB and is formedto a thickness between approximately 3 and 300 angstroms. The insulatingtunneling layer is preferably a layer of Al₂O₃, ZrO₂, AlN, HfO₂ ormultilayers thereof and said tunneling barrier layer is formed to athickness between approximately 3 and 30 angstroms A layer of metallicantiferromagnetic material (45), having the optimal thickness disclosedbelow, is formed on top of the free layer. This “top” antiferromagneticlayer, upon a subsequent annealing in an external magnetic field, willprovide magnetic anisotropy within the plane of the free layer withoutsubstantial exchange coupling. The top antiferromagnetic layer (45) canbe a layer of IrMn, RhMn, RuMn, OsMn, FeMn, FeMnCr, FeMnRh, CrPtMn,TbCo, NiMn, PtMn or PtPdMn and it is preferentially formed to athickness between approximately 2 and 20 angstroms, within which rangethe magnetic anisotropy of the free layer is found to be optimized. Acapping layer (47), which can be a layer of Ru, Rh, Ti, Ta, NiCr,NiFeCr, Cr, Cu, Au or Ag, is formed on the top antiferromagnetic layerto protect the top antiferromagnetic layer from oxidation or otherprocess damage during subsequent processing steps. The pinned layer (60)can be a single magnetic layer or, as is the case in this preferredembodiment, a synthetic antiferromagnetic (SyAF) multilayer, comprisinga first ferromagnetic layer (62) and a second ferromagnetic layer (66)separated by a coupling layer (64) formed of non-magnetic couplingmaterial such as Rh, Ru, Cr or Cu. 5. Ferromagnetic layers (62) and (66)suitable for the objects of this preferred embodiment are layers of Co,Ni, Fe or their alloys or CoFeB, formed to thicknesses betweenapproximately 5 and 100 angstroms. An appropriate coupling layer is alayer of Ru, formed to a thickness between approximately 7 and 8angstroms or a layer of Rh formed to a thickness between approximately 5and 6 angstroms.

The magnetizations of the first and second ferromagnetic layers arecoupled in antiparallel directions and pinned by a “bottom”antiferromagnetic layer (70) formed of the same antiferromagneticmaterial as used in the top antiferromagnetic layer, but with adifferent range of thickness so as to provide a pinning mechanism due tomagnetic exchange coupling. The bottom antiferromagnetic layer is formedon a seed layer (75), for improvement of its structure. The seed layermaterial suitable for this embodiment can include NiFe, NiCr, NiFeCr,Cu, Ti, TiN, Ta, Ru, or Rh. The seed layer is shown as being formed on alower conducting electrode (35), which is required for read operations,but whose structure and composition is not an essential part of thepresent invention.

The material composition and thicknesses of the first and secondferromagnetic layers of the pinned layer (60), as noted above, arechosen so that their magnetizations are essentially equal in magnitude.Thus, when they are fixed in opposite directions, the net magneticmoment of the pinned layer is substantially zero. It is understood thatthe fixing of the SyAF magnetizations and the pinning of the SyAF pinnedlayer to the bottom antiferromagnetic layer is achieved by an annealingprocess in an external magnetic field. It is also understood that thissame annealing process also serves to couple the top antiferromagneticlayer to the free ferromagnetic layer by a magnetic interaction acrossthe interface between the aforesaid two layers, said coupling providingthe free layer with a magnetic anisotropy. Annealing parametersconsistent with the preferred embodiment include an external magneticfield between 100 and 20,000 Oe, applied at a temperature between 100°and 400° C. for a time between 0.5 and 20 hours. It is furtherunderstood that the material and thickness of the top antiferromagneticlayer has been chosen to optimize the coercivity of the free layer whileproviding it with a substantially zero unidirectional magnetic field.Since the MTJ of the type described herein requires no shape anisotropy,it can be patterned so that its horizontal cross-section has asubstantially circular shape, which in the preferred embodiment isapproximately 1.0 microns or less in diameter. In accord with theobjects of the present invention, such patterning can be done usingphotolithographic and ion-milling or reactive ion etch (RIE) methodswell know to those skilled in the art, wherein the photolithographicmask is square or circular. Patterning is done preferentially aftercompletion of the annealing process.

Referring to FIG. 1 b, there is shown the device (cell) of FIG. 1 a, inan overhead view, showing the circular horizontal cross-section of thedevice and the orthogonal intersection of the bit and word lines aboveand below the device.

As is understood by a person skilled in the art, the preferredembodiment of the present invention is illustrative of the presentinvention rather than being limiting of the present invention. Revisionsand modifications may be made to methods, processes, materials,structures, and dimensions through which is formed an MTJ device havinga free layer with magnetic anisotropy provided by magnetic coupling withan antiferromagnetic layer, while still providing an MTJ element havinga free layer with magnetic anisotropy provided by magnetic coupling withan antiferromagnetic layer, formed in accord with the present inventionas defined by the appended claims.

1. A method for forming an MTJ memory cell having a substantiallycircular horizontal cross-section, wherein a ferromagnetic free layer insaid cell has uniaxial magnetic anisotropy provided by exchange couplingwith a top antiferromagnetic layer formed thereon comprising: providinga substrate; forming on said substrate a layered magnetic tunnelingjunction (MTJ) structure, said formation further comprising: forming onsaid substrate a seed layer; forming on said seed layer anantiferromagnetic pinning layer; forming on said antiferromagneticpinning layer a synthetic antiferromagnetic (SyAF) pinned layer; formingon said pinned layer a tunneling barrier layer; forming on saidtunneling barrier layer a ferromagnetic free layer; forming on saidferromagnetic free layer said top antiferromagnetic layer; forming onsaid top antiferromagnetic layer a capping layer; annealing said layeredMTJ structure in an external magnetic field, thereby pinning said SyAFlayer and exchange coupling said top antiferromagnetic layer to saidferromagnetic free layer to produce, thereby, a uniaxial magneticanisotropy in said free layer; patterning said layered MTJ structure tocreate a horizontal cross-sectional shape that is substantiallycircular.
 2. The method of claim 1 wherein said seed layer is a layer ofNiFe, NiCr, NiFeCr, Cu, Ti, Ta, Ru, Rh, TiN, TiW, W or TaW formed to athickness between approximately 5 and 500 angstroms.
 3. The method ofclaim 1 wherein said antiferromagnetic pinning layer is a layer of theantiferromagnetic material IrMn, RhMn, RuMn, OsMn, FeMn, FeMnCr, FeMnRh,CrPtMn, TbCo, NiMn, PtMn or PtPdMn and it is formed to a thicknessbetween approximately 40 and 400 angstroms.
 4. The method of claim 1wherein said SyAF pinned layer is formed by a method further comprising:forming a first layer of ferromagnetic material on saidantiferromagnetic pinning layer; forming a coupling layer on said firstferromagnetic layer; forming a second layer of ferromagnetic material onsaid coupling layer.
 5. The method of claim 4 wherein said first andsecond ferromagnetic layers are layers of Co, Ni, Fe or their alloys orCoFeB, formed to thicknesses between approximately 5 and 100 angstroms.6. The method of claim 4 wherein said coupling layer is a layer of Ru,formed to a thickness between approximately 7 and 8 angstroms or a layerof Rh formed to a thickness between approximately 5 and 6 angstroms. 7.The method of claim 1 wherein said tunneling barrier layer is a layer ofAl₂O₃, ZrO₂, AlN, HfO₂ or multilayers thereof and said tunneling barrierlayer is formed to a thickness between approximately 3 and 30 angstroms.8. The method of claim 1 wherein said ferromagnetic free layer is alayer of Co, Ni, Fe or their alloys, CoFeB, CoZrB, CoTaB or CoHfB formedto a thickness between approximately 3 and 300 angstroms.
 9. The methodof claim 1 wherein said top antiferromagnetic layer is a layer of IrMn,RhMn, RuMn, OsMn, FeMn, FeMnCr, FeMnRh, CrPtMn, TbCo, NiMn, PtMn orPtPdMn and it is formed to a thickness to optimize the uniaxialanisotropy of the ferromagnetic free layer.
 10. The method of claim 9wherein said top antiferromagnetic layer is a layer of IrMn, RhMn, RuMn,OsMn, FeMn, FeMnCr, FeMnRh, CrPtMn, TbCo, NiMn, PtMn or PtPdMn formed toa thickness between approximately 2 and 20 angstroms.
 11. The method ofclaim 1 wherein said annealing comprises raising the MTJ structure to atemperature between approximately 100° C. and 400° C. for a time betweenapproximately 0.5 and 20 hours in an external magnetic field betweenapproximately 100 and 20,000 Oe.
 12. The method of claim 1 wherein saidpatterning produces a circular horizontal cross-section with a diameterof approximately 1.0 microns or less.
 13. The method of claim 1 whereinsaid substrate is a planarized layer of insulation containing therein aconducting word line and wherein said MTJ structure is formedsubstantially over said word line.
 14. The method of claim 13 wherein abit line is formed over said MTJ structure in a direction orthogonal tosaid word line.